The intensive study of molecular events leading to cellular
transformation in tissue culture or in intact organisms culminated in
the identification of 100 or more genes that can be defined as oncogenes
or tumor suppressor genes. Functionally, these genes can be divided into
several classes, each involved in a different step in transmission of
signals from the exterior of the cell to the nucleus. The first
oncogenes to be biochemically character- ized included membrane
receptors for growth factors, growth factors themselves, protein kinases
or small GTP binding proteins involved in signal transduction. Later,
the development of techniques to study pro- teins-DNA interaction in
eucaryotes and the isolation and characterization of many promoter and
enhancer sequences revealed that a number of the classical retroviral
oncogenes were indeed transcription factors. In paral- lel, the rapid
progress in the identification and cloning of chromosomal translocations
in human and animal malignancies and the increased reper- toire of known
transcription factors families revealed that many other transcription
factors can playa critical role in cancer. A more recent devel- opment
concerns tumor suppressor genes. The realization that human tumors are
frequently associated with a loss of function of one or several genes is
also one of the landmarks of cancer research in the last 15 years.
Again, as we will see below, some of these genes encode transcription
factors. It is becoming increasingly difficult to cover in a single
monograph all oncogenes and tumor suppressor genes.